Over many years suggestions have been made to improve the uniformity of a sliver delivered by a card or a drawframe and to keep very closely to a required sliver count. Maintaining a predetermined sliver count at the exit of the card or drawframe is very important for subsequent production of the yarn. The quality and value of the spun yarn depend upon its uniformity, and this can be impaired considerably by failure to maintain the required sliver count after the card or drawframe. Variations in sliver count may lead to frequent interruptions in subsequent processing, leading to expense and impairment of the value of the spun yarn.
Many suggestions have already been made for controlling a card (or the stock chute feed usually disposed before the card) to ensure that the card delivers a very uniform sliver. For example, West German patent document DE-AS 1 069 510 discloses the provision of a vibrator on the vertical chute of a stock chute feed in order to maintain the filling height in the chute permanently at a level such that a downwardly increasing pressure is exerted on the fiber material by weight of the column of fibers. The aim of this is to achieve a substantial uniformity of the sliver by creating a definite exit condition in the chute and thus to avoid substantial variations of the sliver count.
DE-AS 1 918 544 (counterpart of U.S. Pat. No. 3,821,833) proposes a method of pneumatic feeding of flock to a card in which suspended fiber flocks are conveyed by a stream of conveying air through a conveying line to the filling chute of the stock chute feed and deposited therein. The distinguishing feature of this method is that the moisture content of the conveying air is adjusted. In dependence upon the measurement of humidity, a small quantity of air (which has been separated from the conveying air stream, is free from flocks and water droplets and has been conditioned by mixing with water or water vapor) is added to said stream. This method, which already represents a closed-loop control, has as one of its aims to control humidity in the conveying system and thus obviate disturbing agglomerations of flock, so that the conveyed fiber material is deposited uniformly in the chute and the count of the sliver delivered by the card is beneficially affected.
DE-OS 2 050 111 (counterpart of U.S. Pat. No. Re. 27,967) also relates to a closed-loop control aimed at ensuring a very uniform stock at the exit of the chute of a stock chute feed. To this end, the pressure gradient acting on the column of flock is varied, in dependence upon a deviation measured on the passing fiber structure delivered by the chute, from the reference weight per unit of length. Such regulation of the pressure gradient tends to counter the measured deviation.
Another form of closed-loop control of flock feeding is suggested in DE-OS 2 031 788. It is known from this West German patent document to move the lap delivered by the take-off rollers at the bottom end of the chute over a weighing facility in order to ascertain the deviation from a reference weight. The deviation is then used to vary the speed of a motor driving inter alia the take-off rollers.
DE-AS 2 359 917 (counterpart of U.S. Pat. No. 4,030,635) relates to an improvement in the control method described in DE-OS 2 050 111 (counterpart of U.S. Pat. No. Re. 27,967), in which the pressure acting on the flocks in the chute is also controlled in order to inhibit variations in the delivered lap. The control method is so devised as to eliminate the effect of manufacturing and/or assembly tolerances in the measuring rollers used to measure lap thickness. The deviations detected are integrated at time intervals coinciding with the time taken for one complete revolution of one of the delivery rollers, the pressure gradient in the chute being controlled in dependence upon the integrated values obtained.
DE-PS 2 506 061 (counterpart of U.S. Pat. No. 4,133,455) describes a weighing facility suitable for determining deviations of the lap from the set or reference value in a method according to DE-OS 2 031 788.
European application 87 118 415 describes an ingenious open-loop control of a card in which a signal dependent on the thickness of the lap fed to the card is used to control the feed roller preceding the taker-in of the card. A similar arrangement, but in the form of a closed-loop control of the card, is described in the simultaneously filed European application 87 118 414.9 in which a signal dependent upon lap density is produced at the card exit and is additionally taken into account in the control of feed roller speed.
Despite all these steps, it is still possible for the sliver weight at the exit of the card or of a subsequent drawframe not to be completely uniform over a prolonged period of time--i.e., it is impossible to maintain a predetermined sliver count completely accurately over a prolonged period of time.